This invention relates to a helical scan type video data playback apparatus, and more particularly to a video data playback apparatus capable of satisfactorily playing back or reproducing video data recorded on a magnetic tape by the low frequency band color recording system.
In a conventional helical scan type magnetic playback apparatus including a plurality of magnetic heads arranged on a rotary drum, an approach is employed to alternately switch every field playback video signals from the plurality of magnetic heads, e.g., two magnetic heads in dependency upon a rotational position of the rotary drum to play back a switched video signal. Such a switching is called a "head switching". Playback based on this switching is conducted to play back video signals from two successive tracks in a manner that they overlap with each other by winding a video tape onto the rotary drum over an angular range of 180-odd degrees to switch them in synchronism with the detection of a rotational position of the rotary drum. This switching is conducted at the time of playing back the lower part of a frame so that noises on a picture due to the switching become inconspicuous.
However, because a tension of a video tape at the time of recording and that at the time of playback are different from each other, or for other reasons, the following problems exist: One is skew on a playback frame, i.e., shift of a horizontal position of an image of an object played back or reproduced on a frame from the point where head switching is conducted (switching point); the other is shift of the switching point in dependency upon a tracking adjustment when that adjustments is conducted. Further problem is as follows. Since televisions manufactured, etc. are subjected to various adjustments so that a viewer can observe the lower part of a picture as a result of an improvement in recent televisions and video monitors, noises on a picture due to the above-mentioned switching become conspicuous. For example, the switching point is ordinarily set at 6.5 H (horizontal scanning interval) before the vertical synchronizing signal. However, there are instances where the switching point is shifted therefrom about .+-.3 H. Since the switching position appears within a frame at 8 to 4.5 H immediately before vertical synchronization, the place where head switching is conducted would enter that frame.
Additional problem is as follows. At the time of playing back a video tape recorded by other VTRs, the switching point might shift depending upon at which position in a width direction of the tape a video signal is recorded, resulting in that the switching point enters further inside of the frame.